Anyone wishing to play this (or indeed any other Japanese text-based PC game) may find Anime Games Text Hooker
of use. It allows you to hook into the game and extract the text to the clipboard on-the-fly as it is displayed, where you can then use translation software like ATLAS or a translation website like Google Translate or Babelfish to translate it. It's not a perfect solution, it does take some fiddling to set it up, and obviously the translations are not 100% perfect but it's usually good enough to be able to understand what is going on. I personally have used this method to play through the Fate and Unlimited Blade Works scenarios of Fate/Stay Night, but YamiBou is definitely on my list to check out.